Some sulfur deposits are naturally associated with volcanic regions and early sulfur mines were often located in volcanic areas such as Sicily. The Sicilian method of extracting sulfur originally consisted of piling the sulfur bearing rock into large mounds which were then ignited at the top. The heat of combustion of the sulfur in the ore caused underlying layers of sulfur to melt and melted sulfur was collected in molds. This process was very time consuming and inefficient, i.e., sulfur recovery was only about 50%, and copious amounts of obnoxious fumes were produced. The Gill gas furnace, introduced in 1880, moved the sulfur burning operation into chambers and improved the recovery to as high as 80%. Still sulfur wastage was excessive and atmospheric pollution remained severe.
In 1902, the well-known Frasch Process was commercialized and large quantities of very pure sulfur began to be produced by this process from the salt dome deposits of Louisiana and Texas and, later, from other types of elemental sulfur deposits in Mexico, Poland and Iraq. The Frasch Process was very efficient and inexpensive sulfur produced by the Frasch Process and later, from a number of by-product sulfur sources, i.e. sour natural gas, crude oil, etc., eventually largely replaced sulfur Production from deposits of volcanic origin. In recent years however, the exhaustion of many sulfur deposits amenable to the Frasch Process coupled with a steady increase in sulfur demand has led to a renewed interest in the volcanic origin sulfur ores and in other low grade surface deposits of elemental sulfur.
Although volcanic deposits of sulfur occur in many parts of the world and constitute a potentially valuable resource, most of the deposits are not worked. There are several reasons for this. First, the deposits often occur in isolated areas and at high altitudes where transportation and other logistics problems are prohibitive. Because of the inaccessible location, industry has been slow to commit funds to develop the deposits. Second, the much used, efficient Frasch Process cannot be used to mine many of the deposits because the deposits often occur at shallow depths and the Frasch Process requires several hundred feet of overburden. In addition, the finely disseminated nature of the sulfur does not easily coalesce in the Frasch process. Third, the presence of fine gangue in the sulfur has made it difficult to produce a sulfur of the quality usually produced by the Frasch Process. Fourth, much of the sulfur in some deposits is in the form of "sulfides", i.e., iron pyrites, which are particularly difficult to recover. The present process is intended to recover elemental sulfur; not pyritic sulfur.
Many extraction processes other than the Sicilian method and the Frasch Process have been attempted over the years and have met with limited success. The processes have included thermal methods such as distillation and vaporization, flotation, solvent extraction, filtration and a combination of several of these processes. U.S. Pat. Nos. 3,838,979 and 3,102,792 describe thermal methods. U.S. Pat. No. 3,634,046 describes a filtration method for separating molten sulfur from gangue. U.S. Pat. Nos. 2,841,536, 3,512,943, 3,607,143 and 2,798,034 describe solvent extraction processes for recovering sulfur from volcanic and other surface deposits. None of the processes described in the aforementioned patents have been extensively used in industry and all suffer when compared economically with the Frasch Process.
Various combinations of autoclaving, filtration, and centrifuging are used in some processes for recovering sulfur. One such process, elements of which were first disclosed in U.S. Pat. No. 2,537,842, transfers an aqueous slurry of ground volcanic ore to a pressurized vessel or pipe reactor in which heat and agitation cause sulfur particles to melt and agglomerate. The agglomerated sulfur particles are then quenched and the quenched sulfur is separated from remaining gangue by froth flotation, melting and filtration steps. While the process of the '842 patent represents an improvement over any existing processes for recovering volcanic sulfur, it still suffers from relatively high operating costs due partly to inefficiencies in the froth flotation step. The process also requires that ore fed to the process be ground to a fine particle size which increases the cost of the operation and the difficulty in separating the sulfur from the gangue.
A recurring problem in processes developed for recovering elemental sulfur from volcanic ore has been the difficulty of separating the liberated sulfur from the fine ore gangue generated by the processes. The process described herein addresses that problem.
Although the preceding discussion concentrated on elemental sulfur ore deposits of volcanic origin since numerous ore deposits of volcanic origin exist, the discussion is applicable to elemental sulfur ores of non-volcanic origin which lie on or near the surface and which are not amenable to development by the Frasch Process. The process of the present invention may also be used to recover sulfur from waste products which contain sulfur in disseminated form, for example, filter cake from sulfur filtration operations.
There is a need in the sulfur industry for a process which can economically recover elemental sulfur from volcanic ores. The need will increase as Frasch Process-amenable ore deposits become depleted.
It is therefore an object of the present invention to provide an economical process for the recovery of elemental sulfur from volcanic and non-volcanic ores which is economical and efficient. These and other objects and advantages of the invention will be apparent from the description which follows.